I see that there's no mention of this request on the gnulib mailing list by Alan yet and no further comment from Alan here. Alan, are you still thinking about this, or are you planning to bring this topic up with the gnulib developers?

It looks to me as if, in order to do this, you would need to update the implementation of opendirat() in gnulib's file lib/fts.c.

Unfortunately, it's not a given that fts() will always need to be restrained in terms of its effect on the page cache. So, I don't know what the developers will say. But, best to start by asking them at bug-gnulib@gnu.org.